FIG. 1 shows a ducted fan gas turbine engine 10 comprising in axial flow series: an air intake 12, a propulsive fan 14 having a plurality of fan blades 16, an intermediate pressure compressor 18, a high-pressure compressor 20, a combustor 22, a high-pressure turbine 24, an intermediate pressure turbine 26, a low-pressure turbine 28 and a core exhaust nozzle 30. A nacelle 32 generally surrounds the engine 10 and defines the intake 12, a bypass duct 34 and a bypass exhaust nozzle 36. The engine has a principal axis of rotation 31.
Air entering the intake 12 is accelerated by the fan 14 to produce a bypass flow and a core flow. The bypass flow travels down the bypass duct 34 and exits the bypass exhaust nozzle 36 to provide the majority of the propulsive thrust produced by the engine 10. The core flow enters in axial flow series the intermediate pressure compressor 18, high pressure compressor 20 and the combustor 22, where fuel is added to the compressed air and the mixture burnt. The hot combustion products expand through and drive the high, intermediate and low-pressure turbines 24, 26, 28 before being exhausted through the nozzle 30 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines 24, 26, 28 respectively drive the high and intermediate pressure compressors 20, 18 and the fan 14 by concentric interconnecting shafts 38, 40, 42.
In operation the different turbine stages are loaded rearwards. This loading is at least partially offset by the respective compressor which is loaded forwards, with the net rearward force being taken up by various supports structures via the bearings. However, in some instances, the net rearward force may load the bearings to an undesirable level causing premature wear and potential lifting issues.
In the past, existing arrangements have provided a seal panel which acts as a pneumatic piston which is used to provide a forward loading on the fan to offset the rearward loading on the turbine. GB2323637, GB2444935 and GB2461778 all show these sorts of arrangements.
Referring to the applicant's own previously published application GB2323637, although not described in the document, the carrier 44 and seal 42 provide a sealed chamber which can be pressurised with air from a compressor. This air is at a higher pressure than the ambient air which surrounds the fan hub on the upstream side of the seal panel. Thus, in use, the relatively high pressure air provides a forward load or bias on the seal panel once the compressor which supplies the high speed air is up to speed. The level of forward loading increases with the compressor speed which is matched to the turbine speed and relates to the rearward loading on the turbine. Thus, the harder and faster the turbine is driven, the greater the rearward force, but the greater the compressor speed and air pressure provided to the seal panel.
The radial location of the seal is determined in part by the amount of forward loading required from the seal panel. The greater the radial distance of the seal from the principal axis of the engine, the greater the area of the seal panel and the greater the forward loading of a given pressure of air. In the example shown, the level of forward loading is significant and so the radial location is towards the casing. Placing the seal at such a radial extent requires a large seal due to the resultant circumferential length. When using a large conventional seal this inevitably comes with a weight penalty.
The present disclosure seeks to provide an improved seal panel.